Hi,
Recently I came across an issue regarding bad HF dynamic impedance values after a certain stage of production.
The bq78PL116 is powered for a short period of time on a PCA where measurements are done to test components such as FETs and so on. The bq78PL116 is flashed with the (.dat) file and the PCA is checked. In this stage of production, testing generated large HF dynamic impedance values for the 'cells' which are of course not yet present. (pack is assembled at another location)
NO cycle is done after assembly, just a short load pulse to check cells. Once assembled 20% of the packs are 'dead' within 2 weeks. It has been traced back to the balancing circuits transferring charge between cells back and forth...due to OCV fluctuations and the selected balancing algorithm.
Configuration information:7S3P - Lithium-Ion
MY QUESTIONS ARE:
- Why is there a fluctuation in OCV when a high HF value is present? What is the correlation between the HF values and OCV predictions?
- I’ve noticed in some literature OCV=f(SOC, temperature), but this does not explain the fluctuation in OCV.
- OVC values fluctuate by as much as 20mV. Making balancing circuits essentially 'go crazy'.
- Would setting the 'wired bit' to 0 prevent HF value updates during testing?
- Setting the balancing algorithm to use SOC also causes the same problem, why?
The only way to clean it up is to pulse the battery pack till the HF values are updated for all cells. Do you have any recommendations for this problem? –Production, settings ect.
Most importantly I need an explanation as to why the OCV is fluctuating for the cells.
Thanks